This study note reviews the basic orbit parameters and characteristics of the four global navigation satellite constellations operational or planned in medium Earth orbit: GLONASS (Russia), GPS (USA), Beidou-M (China) and Galileo (Europe). The launch history, mission profile, spacecraft lifetime and maintenance strategy are detailed, based on the available literature and open information. The disposal rules applied so far to constellation spacecraft and upper stages are discussed, together with their long-term impact in terms of orbital evolution and possible crossing of the altitudes of the operational satellites. Moreover, the average cross section (randomly tumbling) is estimated for some of the spacecraft models of the constellations. Finally, the detailed input setup for the long-term simulations planned in the framework of the study is presented.